When electric motors are used in a machine tool cutting operation, large in-rush currents are temporarily generated when the electric motors are reversed to change the direction of the cutting tools. Heretofore, fuses were used in series with thermal overload relays in the motor control equipment to protect the motor and associated equipment. When the fuses were subjected to thermal aging by repeated subjection to high transient reversing currents the fuses had to be frequently replaced. With the advent of the so-called "energy efficient" motors that operated at lower normal operating currents and higher currents during current reversal, the ratings of the fuses were selected to sustain the higher transient reversing currents and resist thermal aging. The higher-rated fuses however do not coordinate as well with the thermal overload relays used within the motor control units and find limited application outside the tool-making industry.
U.S. Pat. No. 5,153,544 describes an industrial-rated circuit breaker having an electronic trip unit that is adjustable over a wide range of long and short time over-current conditions. The circuit breaker also includes an accessory cover wherein components can be installed within the field without disturbing the circuit breaker operating components.
U.S. Pat. No. 4,967,304 describes a digital circuit interrupter with electric motor trip parameters that includes control algorithms tailored for motor protection applications. It would be economically advantageous to have a combined circuit breaker-fuse arrangement whereby the fuse does not need frequent replacement when used within machine tool cutting operations and wherein the circuit breaker magnetic trip function can be adjusted to provide coordination between the short circuit protection provided by the fuse and the long time over-current protection provided by the thermal overload relay.
One purpose of the invention is to provide a circuit breaker-fuse arrangement wherein the short circuit protection provided by a fuse can be coordinated with the long time over-current protection provided by a thermal overload relay